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Reducing the bit size of bits in C #

I am using C # and have an image saved in a Bitmap object.

Now I would like to convert this image to an 8-bit grayscale, and then to a 4-bit grayscale image.

Do you have any tips on how to do this?

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In .NET Bitmap formats, there is no such thing as an 8 or 4-bit grayscale image. Supported formats are listed using the PixelFormat enumeration . However, you can create a 4 or 8-bit image by creating an indexed image (8bppIndexed or 4bppIndexed), where each entry in the palette represents a gray scale value.

This code takes a bitmap and creates a copy as an 8bpp indexed image with grayscale values:

public static Bitmap BitmapToGrayscale(Bitmap source) { // Create target image. int width = source.Width; int height = source.Height; Bitmap target = new Bitmap(width,height,PixelFormat.Format8bppIndexed); // Set the palette to discrete shades of gray ColorPalette palette = target.Palette; for(int i = 0 ; i < palette.Entries.Length ; i++) { palette.Entries[i] = Color.FromArgb(0,i,i,i); } target.Palette = palette; // Lock bits so we have direct access to bitmap data BitmapData targetData = target.LockBits(new Rectangle(0, 0, width,height), ImageLockMode.ReadWrite, PixelFormat.Format8bppIndexed); BitmapData sourceData = source.LockBits(new Rectangle(0, 0, width,height), ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb); unsafe { for(int r = 0 ; r < height ; r++) { byte* pTarget = (byte*) (targetData.Scan0 + r*targetData.Stride); byte* pSource = (byte*) (sourceData.Scan0 + r*sourceData.Stride); for(int c = 0 ; c < width ; c++) { byte colorIndex = (byte) (((*pSource)*0.3 + *(pSource + 1)*0.59 + *(pSource + 2)*0.11)); *pTarget = colorIndex; pTarget++; pSource += 3; } } } target.UnlockBits(targetData); source.UnlockBits(sourceData); return target; }

To make a 4Bpp image, you need to create a target using PixelFormat.Format4bppIndexed and then set the ColorPalette to 16 discrete shades of gray. Finally, in the loop, you should normalize the values ​​of 2 between 0-15 and pack every 2 pixels in one byte.

This is a modified code to make a 4bpp grayscale image:

  public static Bitmap BitmapToGrayscale4bpp(Bitmap source) { // Create target image. int width = source.Width; int height = source.Height; Bitmap target = new Bitmap(width,height,PixelFormat.Format4bppIndexed); // Set the palette to discrete shades of gray ColorPalette palette = target.Palette; for(int i = 0 ; i < palette.Entries.Length ; i++) { int cval = 17*i; palette.Entries[i] = Color.FromArgb(0,cval,cval,cval); } target.Palette = palette; // Lock bits so we have direct access to bitmap data BitmapData targetData = target.LockBits(new Rectangle(0, 0, width,height), ImageLockMode.ReadWrite, PixelFormat.Format4bppIndexed); BitmapData sourceData = source.LockBits(new Rectangle(0, 0, width,height), ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb); unsafe { for(int r = 0 ; r < height ; r++) { byte* pTarget = (byte*) (targetData.Scan0 + r*targetData.Stride); byte* pSource = (byte*) (sourceData.Scan0 + r*sourceData.Stride); byte prevValue = 0; for(int c = 0 ; c < width ; c++) { byte colorIndex = (byte) ((((*pSource)*0.3 + *(pSource + 1)*0.59 + *(pSource + 2)*0.11)) / 16); if (c % 2 == 0) prevValue = colorIndex; else *(pTarget++) = (byte)(prevValue | colorIndex << 4); pSource += 3; } } } target.UnlockBits(targetData); source.UnlockBits(sourceData); return target; }
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